Magnetic resonance imaging (MRI) technology is used for diagnostic purposes in hospitals and medical research centers. Through the use of two magnetic fields, one static and one time-variant, MRI technicians can obtain very detailed, high resolution images of the human body.
Magnets, such as superconducting magnets, permanent magnets, and electromagnetic magnets, used in applications such as magnetic resonance imaging produce stray static magnetic fields. The magnitude of these stray fields can vary significantly depending on the distance relative to the center of the magnet. The earth produces a magnetic field slightly less than one Gauss, but fields above 5 Gauss can cause electronic devices such as pacemakers to malfunction. In addition, the magnetic field strength of an MRI magnet is often large enough to turn ferrous objects into projectiles and cause electrical devices and magnetic strips to malfunction.
The static magnetic field can extend several meters outwards from an MRI machine cavity at strengths of up to 3 Tesla, or about 3×105 times the strength of Earth's magnetic field. Objects with significant ferromagnetic content in the vicinity of an MRI machine can be attracted to this field and accelerated at high speeds toward the cavity, posing a significant risk of damage to the machine and the threat of injury to patients undergoing scanning. The removal of foreign ferromagnetic objects from an MRI cavity can also require the machine to be shut down completely (quenched), a time-consuming and expensive process.
Others have attempted to provide a safer environment for MRI's to be performed. U.S. Patent Application Publication No. U.S. 2004/0135687 A1 teaches a ferromagnetic object detector that measures the change in a known magnetic field when a ferrous object passes through. The detector incorporates a proximity sensor to trigger the magnetic field detector, reducing the number of false alarms. U.S. Patent Application Publication No. US 2003/0171669 A1 teaches an MRI Protector that incorporates an array of Hall sensors around an entrance to an MRI room.